A basic valve type distributed liberally throughout an industrial refrigeration system is a manual shutoff valve. In the completely open position, this valve should allow a free flow of refrigerant and when closed completely block the flow. The usual function of the shutoff valve is to isolate a component or a section of the system. Some major categories of manual shutoff valves are globe, angle, inline, and ball valves, as shown schematically in Fig. 11.1. Three desirable characteristics of manual shutoff valves are:
– that they permit no passage of refrigerant when closed
– that they cause only a low-pressure drop of refrigerant flowing through them when they are open
– that they do not leak to atmosphere
Several other types of valves are gate and butterfly valves which meet the lowpressure-drop requirement, but in general do not seal as well as other valves when closed. Consequently they are not widely used in industrial refrigeration service. All valves shown in Fig. 11.1 have accessible handles, but in recent years a strong preference has developed for capped valves when the valve does not need to be opened and closed often.
Shutoff valves are oriented so that they close against the flow which usually means that they close against the high pressure. With this orientation the upstream pressure assists in the opening of the valve. If the valve must open against high pressure, cases have been reported where the pressure holds the disc with such force that the stem may pull away from the disc in an attempted opening. When the valve closes against the flow the highest pressure is kept off the stem and bonnet when the valve is shut off. Globe valves should be mounted with the stem horizontal so that any vapor in the line cannot form a pocket at the valve inlet which would periodically release, causing noise and unsteady flow.
Ball valves have become very popular in the past few years, primarily because of the low pressure drop that they cause in their completely open position. A further advantage of ball valves in certain situations is that they are quarter turn valves so that a quarter turn of the handle permits quick opening or closing of the valve. An undesirable characteristic of the basic ball valve is that of trapping liquid within the ball when the valve is shut off. A ball valve in a cold liquid line traps cold liquid inside the ball when the valve is closed, and this liquid is likely to warm up when the flow is interrupted. The trapped liquid expands which could blow out the valve seat or even rupture the valve body.
Two methods used most commonly to relieve pressure of trapped liquid in the ball and prevent damage are upstream-venting and self-relieving seats. In upstream venting a small hole is drilled through one side of the ball, connecting the upstream line with the cavity when the valve is in its closed position. This configuration bypasses the upstream seat, and provides a continuous vent path for cavity pressure. In the self-relieving seat design, the seats act as internal relief valves to open a vent path from the valve body cavity to the line. Self-relieving valve seats serve as normal valve seats unless the pressure within the ball rises to an extreme level, in which case they permit leakage of a few drops of liquid.
Judgment should be used in whether and what type of valves should be incorporated in the lines. Even valves that are rarely shut off may be invaluable in isolating a certain component or even another valve on rare occasions. On the other hand, extra valves, particularly those placed in vapor lines, may represent a persistent demand for extra compressor power when ever the system operates. One estimate2 calculated that a fully open valve in the liquid/vapor line between the evaporator and the low-pressure receiver causing 7.5-kPa (1.1-psi) pressure drop could add $9,400 to the annual operating cost of a 2100 kW (600 ton) system. A fully open ball valve would add only $43 to the annual operating cost.
Some pressure drop is expected when refrigerant flows through open valves, and this pressure drop adds to the pressure drop occurring in straight sections of pipe. Methods for calculating the pressure drop in straight pipes were presented in Chapter 9, and Table 11.1 provides data for computing the pressure drop in fittings and valves. Table 11.1 gives the values of the c-terms in the equation
An important observation that can be made from Table 11.1 is the relative pressure drops of globe and angle valves. An angle valve causes anywhere from 1/2 to 1/8 the pressure drop of a globe valve (depending upon the valve and pipe size) and should be considered if the physical arrangement permits. The pressure-drop coefficients for ball valves are likely to be approximately the same as gate valves.